Adobe Reader/Acrobat Reader - free
Adobe Acrobat Pro/Adobe Acrobat Standard - hundreds of dollars
There are many differences of course but this is the most obvious one.
But I'm guessing this file was "Reader Enabled" which is magic HMRC can do to allow you to save the file. What has happened is that this has been removed, possibly because the file was edited with non-Adobe software (even once), but possibly because of a problem. You have have had a message about "Extended features".